Double voltages on PC power supply output

[Webman]

I normally test PC power supply faults by just swapping parts, I thought I would try and test with a meter after reading an article.
The PSU is a 250w ATX with the 20 pin connector.
I shorted the connectors 'On' with the 'Ground next to it and swithched on - fan starts.
I then placed the red lead of a test meter (set to ACV) in say the 12v pin hole, the black lead on a ground - reading 24.3v
5.5v pin hole - reading 10.8v
3.3v pin hole 6.8v

The meter has two ACV settings 600 and 200, I was using the 200.
Can someone please explain what was wrong - by the way I tried this on a 400w ATX and had the same problem

 

[Nigel Goodwin]

I normally test PC power supply faults by just swapping parts, I thought I would try and test with a meter after reading an article.
The PSU is a 250w ATX with the 20 pin connector.
I shorted the connectors 'On' with the 'Ground next to it and swithched on - fan starts.
I then placed the red lead of a test meter (set to ACV) in say the 12v pin hole, the black lead on a ground - reading 24.3v
5.5v pin hole - reading 10.8v
3.3v pin hole 6.8v

The meter has two ACV settings 600 and 200, I was using the 200.
Can someone please explain what was wrong - by the way I tried this on a 400w ATX and had the same problem

You need the meter set to DC, all the PSU outputs are DC.

 

[Webman]

Thanks very much - how stupid of me.
When I reread the artical it did not state which test setting to use. I just assumed - and very incorrectly.

Regards
Phil

 

[daviddoria]

infact that is the whole point of the power supply... to take the AC and convert it into a very clean DC

all of the computers components are digital, and digital stuff requires nonoscillating power

 

[440uf]

I'd be interested to hear what voltage readings your getting, now you've changed the meter to DCv. The psu that I pulled out of my PC recently is only showing 10.5v on the 12v pins. It shows a healthy 5.65 on what are theoreticaly the 5v pins. I'm asking because the PC was working fine with the supply, but how, with only 10.5v?

 

[Sebi]

The 12V output drive only insensitive devices, HDD-motors, fans.
More problem with 5V, because the control feedback live from this output, so this always 5V, 5,6V not acceptable.
You are lucky, this failure can destroy the complete computer.
Check all condensers on outputs.